WUFI Part 3 – Options

We are often contacted by clients asking for “some Wufi calculations”. Sometimes such enquiries indeed end up with a set of Wufi simulations, and more importantly, a thorough report analysing the outputs, drawing conclusions and making recommendations. But often, we will respond to the enquiry differently, using other methods as well as, or instead of Wufi.

Wufi is a tool with a specific job

Think of Wufi like a tool in your workshop. It has a specific job; it does some things very well, some things not so well and many things, not at all. Other tools may do certain things better – maybe faster or more accurately.

We first tell the software what the build-up is, along with the internal conditions and external weather, lots of other parameters such as angle of inclination, and set a simulation running. We usually simulate at least five years, often ten or more, and estimate the moisture behaviour at hourly intervals. We can extract graphs showing the progression over time of moisture content of specific elements, as well as temperature and humidity. Using these we make judgements against various criteria as to whether there are risks from moisture accumulation such as mould, rotting of organic materials, corrosion and spalling. Unlike the Glaser method the software does not provide a simple pass/fail result.

In general terms, Wufi is good for predicting whether, and where, moisture might accumulate within a construction. In the context of a Glaser calculation, this might be referred to as interstitial condensation, but this is an incomplete phrase. Interstitial refers to what happens between layers of material, i.e. at the interface between, for example, insulation and a breather membrane. The Glaser method indeed considers what happens at each interface, but Wufi simulates what happens throughout each layer as well. The effects of this can be significant where there are materials that can store moisture (and heat) and transport liquid water by capillary action (and other mechanisms). This might sound exotic and only for niche products, but it applies to common materials such as brick and timber.

Wufi can also be used to assess the risk of surface moisture, and subsequent mould growth. We don’t often use this because there are other assessment methods that are adequate and faster.

Wufi may not be the right tool

Wufi is a dynamic hygrothermal simulation tool, which means it makes predictions about the movement of heat and moisture through a construction over time. Setting up a simulation properly takes time, knowledge and skill, and interpreting the results can take even longer and draw on more experience. In some cases, it is a sledgehammer to crack a nut.

For example, if for whatever reason you are most concerned about moisture on the surface of a build-up, it’s probably not necessary to fire up Wufi. If you have a 2D or 3D problem, Wufi Pro won’t be your primary tool, but it may provide some supplementary information, for example in preparation for a Wufi 2D simulation or in tandem with a 2D thermal simulation using Therm or a similar piece of software.

Other relevant tools

As well as Wufi we have other tools in our workshop.

We will often use Therm or Psi-therm for the analysis of thermal bridges. Such tools use a calculation known as Finite Element Analysis, which can be performed in 2D or 3D, depending on the software used. Such simulations give us two pieces of information. Firstly, the rate of heat loss, which is used in SAP, PHPP and other energy modelling tools to account for heat loss.

Secondly, we can obtain the minimum internal surface temperature, which is more relevant to this post. As well as losing more heat, thermal bridges will locally reduce internal surface temperatures. When warm air is cooled down, its relative humidity increases, and this is true of the air adjacent to the internal surface near a thermal bridge. In general terms, mould can start to propagate at 80% RH (depending on temperature), so a thermal bridge may give rise to surface mould. This is one reason why mould is often found in the corners of buildings, where geometrical thermal bridging occurs. Wufi Pro is not useful in such cases because it is a 1D tool so cannot directly model thermal bridging. Wufi 2D may be useful here, but if Wufi Pro is a sledgehammer, then Wufi 2D is a 200 Ton Hydraulic press! In some cases, it may be useful or important to understand what’s happening within a 2D junction, but the number of cases where the work involved is justified is relatively small.

The Glaser method is a simple tool with significant limitations, but sometimes the context is such that we are working within such limitations and it is acceptable to use it. It should therefore not be dismissed out of hand, but only be applied carefully after proper consideration.

Excel is a powerful piece of software which must not be overlooked. It is invaluable for post-processing simulation results and refining input data. It can also be used to create calculation tools from scratch, for example the Glaser method can be implemented right in excel.

A relatively new tool called HTFlux is another candidate for consideration. This is essentially a 2D implementation of the Glaser method. It has the same limitations as the conventional 1D Glaser method, but also is not covered by British Standards or European Norms.

Beyond Wufi 2D, there are a number of commercial ‘multi-physics’ simulation tools to consider. These are more widely used in industries such as automotive and aviation, and typically capable of Finite Element Analysis, Computation Fluid Dynamics or both. To stretch our tool analogy, these would be a cross between a swiss army knife and a JCB – multi-purpose but tricky to drive and rather expensive.

This list is not exhaustive. Sometimes it’s necessary to consider ventilation (mechanical or natural) or infiltration in detail, using principles and calculations which are too numerous and complex to mention here.

Finally, the most important tool we have is our brain, and all the knowledge and experience stored in it. Often we can make recommendations without having to run any simulations. It’s important to carefully study and understand a problem before opening up any of these tools. What, exactly is the problem – where does moisture come from and where does it go?

Conclusions – Horses for Courses

Wufi is one tool amongst many that are relevant when we assess moisture risks in buildings. In many cases it is the most appropriate tool to help solve a problem, but in other instances it is one of a number that must be deployed, and sometimes it’s simply not appropriate. If you have concerns about moisture in a building, talk to us about what the best approach might be to assess and solve the problem.


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